1. Field of the Invention
The present invention relates to integrated circuits, and, in particular, to variable gain amplifiers.
2. Description of the Related Art
The variable gain amplifier (VGA) shown in FIG. 1 has been used for many years and is particularly useful in applications where the input is at a constant amplitude, but the output level must vary over a wide range. The circuit has excellent high-frequency performance and its distortion performance is independent of the gain setting (to first order). Assuming that RC11=RC12=RC1, and RE11=RE12=RE1, it can be shown that the voltage gain A.sub.V of the circuit is given by Equation (1) as follows: ##EQU1## where VAGC1 is the VGA control voltage, T=absolute temperature, k=Boltzman's constant, and q=fundamental electron charge.
A desirable feature of this VGA is that the gain control has an exponential characteristic. An undesirable characteristic is that, when the control voltage VAGC1 is constant with temperature (as is typically the case), the gain of the amplifier will have a strong temperature dependence. The temperature dependence is a result of the kT/q term in Equation (1).